for each expressions can't be negative.   

<startCounterValue expressionLanguage="anyURI"?>
      unsigned-integer-expression
   </startCounterValue>
   <finalCounterValue expressionLanguage="anyURI"?>
      unsigned-integer-expression
   </finalCounterValue>
   <completionCondition>?
      <branches expressionLanguage="anyURI"?
         successfulBranchesOnly="yes|no"?>?
         unsigned-integer-expression
      </branches>
   </completionCondition>

I used the following forEach in a BPEL and the editor didn't validate that the expressions are invalid. All the values
startCounterValue, finalCounterValue, completionCondition are ngative. 

        <forEach name="ForEach1" parallel="no" counterName="svcIdx">
            <startCounterValue>-1</startCounterValue>
            <finalCounterValue>-2</finalCounterValue>
            <completionCondition>
                <branches>-1</branches>
            </completionCondition>
            <scope name="Scope1">
                <sequence name="Sequence1">
                    <empty name="Empty1"/>
                </sequence>
            </scope>
        </forEach>